A predator’s influence extends far beyond the animals it eats; it has the power to change the behavior of any creature it can make wary, according to ecologist Justin Suraci.
Picture a northern Pacific archipelago: forested islets and sandstone beaches, water lapping against a rocky shore. A raccoon ambles around in the intertidal zone in search of food.
Now inject terror into the scene: the sound of a hungry dog barking. Suddenly, the landscape is transformed.
This is what Justin Suraci, an ecologist at the University of Victoria, found after spending several years on the Gulf Islands in British Columbia, trying to terrorize some small mammals. It was a silly-sounding project — and Suraci will admit he looked pretty silly doing it — with a sobering result.
Terror turns out to play a critical role in balancing the ecosystem, Suraci said in a study published Wednesday in the journal Nature Communications. It’s the driving force of a drama that plays out on the Gulf Islands and every other landscape in the world.
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Fear of large predators — not just the predators themselves but the larger, all-encompassing dread of their presence — keeps smaller “mesopredators” in check. It means those animals spend less time eating and more time worrying about getting eaten. This, in turn, allows their even smaller prey to flourish, maintaining an ecological harmony that has been honed for millennia.
But dread is vanishing. Humans have killed off large predators in staggering numbers, bringing wolves, sharks, tigers, bears, lions and others to the brink of extinction — or past it. In doing so, humans destroyed not just individual species but an entire “landscape of fear,” Suraci says, changing the natural world for the worse.
Here’s why he thinks that.
The importance of large carnivores — known as “apex predators”— in ecosystems is well-known. A famous study in Yellowstone National Park, where gray wolves had been missing for decades before being reintroduced in the 1990s, showed that the presence of that single, irreplaceable predator set off a series of cascading positive effects in the environment. The elk that had been preventing willows and aspen from taking root on the riverbanks were brought into check. Migratory birds started roosting in the recovered trees, and trout began swimming in the shady waters beneath their branches. Beaver colonies boomed, fed by the low-hanging willow branches, and their dams created marshes where otters, mink, muskrats and ducks could nest.
“For the first time in 70 years, the park has a complete suite of predators and prey,” Oregon State University forest ecologist William Ripple, a co-author on the study, told The Washington Post in 2004. “This is a grand experiment.”
The explanation for apex predators’ outsized influence on their landscapes has always relied on their regulatory role: By hunting mesopredators, the way Yellowstone’s wolves hunt elk, the apex predators maintain a balanced ecosystem.
But some ecologists believe the equation is more complex. The mere fact of hunting doesn’t always keep a pesky population in check; just look at American white-tailed deer, whose population growth has defied human hunters, much to many gardeners’ chagrin.
Meanwhile, lab experiments, mostly involving tanks full of fish and small invertebrates, have shown that animals will alter their behavior when they think there are predators around even if the predator never shows up. It didn’t necessarily take a mesopredator being eaten for a landscape — or in these cases, a fish tank — to change. It just took a mesopredator being aware of the possibility.
That makes sense to Suraci.
“Any predator is only ever going to eat some of its prey,” he said. “But it’s potentially going to scare all of its prey, and cause all of its prey to change its behavior all of the time.”
In other words, a predator’s influence extends far beyond the animals it eats; it has the power to change the behavior of any creature it can make wary. Wary animals spend less time eating, because they’re too busy hiding, or scurrying, or jumping at loud noises, keeping their heads up and their ears attuned to the approach of anything that might find them appetizing. And that can have the same cascading effects as if the mesopredator really had been eaten. In biological terms, being fearful may change a creature’s impact on the landscape as much as being dead.
But it’s difficult to test this theory. Ecosystems are big, complex arrangements, and it’s hard to separate the fact of predation from the fear of it when looking at the cascading impact of a predator’s presence. Besides, you can’t ask the Yellowstone elk if they’ve moved out of the riverbeds because they’re worried a wolf might spot them there and decide it’s time for his next meal.
So Suraci manufactured fear.
His victims were the rampaging raccoons of the Gulf Islands. The animals had flourished basically unchecked since their natural predators — cougars, bears — were killed off by humans. They flocked to the exposed rocks of the islands’ intertidal zone in search of tasty meals of fish and crab, and they showed none of the vigilant behavior one might expect of a medium-size mammal. They even hunted during the day, something raccoons with a better sense of their own mortality never do.
Suraci and his colleagues reminded them of how easily they could be eaten, setting up speakers in the trees near the tidal zone and programming the speakers to play sounds of dogs barking. (They also set up a control speaker system on another island that played sea-lion sounds, to ensure that it was the dogs, and not the noise, that affected the raccoons’ behavior.) Then they sneaked around the woods and shoreline with video cameras trying to capture the raccoons’ response, like the producers of some bizarre reality show.
“It was one of the most fun jobs I’ve ever had,” Suraci said.
The results were striking. Not only did the raccoons spend far less time in the exposed intertidal zone, where they could have been easily seen and hunted by a predator (if it existed), when they were there, they spent much more time scanning their surroundings than with their faces down, looking for food. All told, the raccoons spent 66 percent less time foraging than when they were unafraid of being eaten.
The effects of that shift were apparent across the landscape they inhabited. The populations of two types of raccoon prey — intertidal crabs and intertidal fish — nearly doubled, and two other species that were being eaten by raccoons increased by more than 50 percent. Meanwhile the number of staghorn sculpin (a crab competitor that had flourished when all the crabs were being eaten) and periwinkle snails (crab prey) fell significantly.
The ecosystem had been transformed. Suraci and his colleagues had restored the landscape of fear.
“I think it does suggest that fear is in some respects a critical component of ecosystems,” he said. “When you remove the top level of the food chain as humans have done across the globe … you don’t just remove the actual predation but you remove the behavioral effects as well.”
Bringing fear back, he continued, “restored a balance to the food chain that had been lost when the top level was taken away.”
Real source of fear
But the solution is not for ecologists to go around terrifying small mammals. Suraci only played his dog noises in the Gulf Islands for a month; if he had kept going, eventually the raccoons would realize that the sounds didn’t correspond to a real threat, and they’d stop being afraid.
No, we need to restore the real source of fear. And that means overcoming our own.
“It’s interesting,” Suraci said. “The very fear that we’re suggesting is so beneficial to ecosystems is the same as the fear that [caused] carnivores to get killed off in the first place.”
Humans don’t like lions, wolves, cougars and sharks any more than raccoons and elk do — at least, not up close. We fear what will happen if they get too close to our homes, our livestock, our children. And though there are myriad reasons that humans have brought about the decline of apex predators — habitat destruction, competition for prey, slaughtering them for fashion and sport — the fact that we’re compelled by fear to kill them before they can kill us is certainly a big one.
But we’ll have to get over it, Suraci says, if we ever want to return landscapes to the healthier, richer — and scarier — places they once were.